As semiconductor devices are scaled down to smaller feature sizes, interconnect delay becomes a significant performance barrier for high-speed signal conduction. The use of dielectric materials with a lower dielectric constant (k) can significantly improve performance measures by reducing signal propagation time delay, cross talk, and power consumption in semiconductor devices having a multilevel interconnect architecture. The most-used dielectric material for semiconductor fabrication has been silicon oxide (SiO2), which has a dielectric constant in the range of k=3.9 to 4.5, depending on its method of formation. Dielectric materials with k less than 3.9 are classified as ‘low-k dielectrics’. Some low-k dielectrics are organosilicates formed by doping silicon oxide with carbon-containing compounds. One such low-k dielectric material is methylsilsesquioxane (MSQ), which is a polymer having an inorganic backbone of alternating silicon and oxygen atoms with methyl (CH3) groups attached to the silicon by a silicon-carbon (Si—C) bond. MSQ typically has a dielectric constant in the range k=2.6–2.8.
Well-known steps in the fabrication of a semiconductor device include forming a layer of dielectric material on a silicon wafer or on previous levels of the device, applying a layer of photoresist material to the surface of dielectric material, exposing areas of the photoresist layer to light through a pattern mask, dissolving the exposed areas of photoresist with a developer solution to reveal the underlying areas of dielectric material, etching the revealed areas of dielectric material, removing the remaining photoresist layer, and filling the etched dielectric material with metal.
The steps of etching the dielectric material and removing the photoresist layer may be performed with an O2-containing plasma, which can degrade the dielectric properties of the dielectric material through oxidation. This damage to the material is believed to occur when Si—C bonds are broken and hydrophilic hydroxyl (OH) groups replace the hydrophobic methyl groups in the MSQ. The polarity of the dielectric material is thus changed and the damaged dielectric more easily absorbs moisture, resulting in an increase of both leakage current and dielectric constant. Subsequent heating of the damaged dielectric material can release moisture, interfering with the process of filling the etched cavities with metal. Semiconductor devices fabricated with such damaged dielectric material exhibit reduced performance measures and increased fabrication defects compared to devices fabricated with undamaged dielectric material.
Degradation of the dielectric properties of MSQ through substitution of hydroxyl groups for methyl groups may occur in other processing steps as well, such as creating nanopores in the dielectric layer by forming it from a composite of MSQ and triblock copolymer, e.g., poly(ethylene oxide-b-propylene oxide-b-ethylene oxide), and calcinating the block polymer to generate pores. Another processing step, chemical-mechanical polishing, can also produce increased leakage current and dielectric constant in MSQ. The kinematic mechanical abrasion and a chemical reaction with polishing additives such as tetramethylammonium hydroxide can break the Si—C bonds, resulting in moisture uptake and dielectric degradation.
Experiments have shown that treating the damaged dielectric material with trimethylchlorosilane (TMCS) ((CH3)3Si—Cl) reduces the leakage current and dielectric constant of the material. This is thought to result from a reaction in which the TMCS replaces a hydrophilic hydroxyl group with hydrophobic trimethylsiloxane ((CH3)3SiO), causing a decrease in the moisture uptake of the treated dielectric. Treatment with hexamethyldisilazane (HMDS) ((CH3)3Si—NH—Si(CH3)3) has also been shown to reduce leakage current and dielectric constant. This is thought to result from a reaction in which two hydroxyl groups are replaced with trimethylsiloxane. These are relatively inefficient chemical replacement reactions, however, which require high pressures or long treatment times to achieve their results. Furthermore, while replacing the hydrophilic hydroxyl groups with hydrophobic methyl groups, these treatments have not been shown to restore the silicon-carbon bonds and, as a result, the polarity of the damaged dielectric material.
Alternatively, the damaged dielectric material can be treated with hydrocarbon, fluorocarbon, or organo-substituted silane gases (e.g., (CH3)xSiH(4-X), where x is 1 to 4). This treatment has been shown to reduce defects in metal fillings deposited on the treated dielectric material. However, the effect of this treatment on the dielectric properties of the damaged dielectric material has not been demonstrated.